Changes in expression of the principle opioid peptide gene, preproenkephalin, follow stimulation or inhibition of inputs to antagonist drugs. The duration of proenkephalin gene up- or down-regulation can outlast the duration of the stimulus, suggesting that regulated gene expression could provide one substrate for storing information about previous stimulus or drug exposure. This would have direct implications for possible mechanisms of opiate tolerance and withdrawal. We have thus sought evidence for functional up-regulation of enkephalinergic gene regulation systems in humans. The subjects were 19 males with no reported histories of opiate abuse. Pain self-report to a 5 min ice-water immersion and cold-stress-induced analgesia were tested before 8 days of antagonist treatment, 2 days after the last dose of antagonist (when excretion of antagonist metabolites was finished) or placebo, and finally 30 min after an acute oral dose of naloxone. Self-report of pain to the initial stages of the ice-water immersion were significantly reduced in subjects 2 days after opiate antagonist treatment. Initial results of the study provide evidence for increased function in endogenous opioid systems after antagonist washout. Current studies aim to separate pre- and post-synaptic components to this effect. We studied 30 normal males in a second study that was a replication and extension of the first. The procedure was similar, with the addition of an oral hydromorphone challenge at the end to: 1) quantify the degree of analgesia to a mu-type opiate and 2) rule out post-synaptic changes as an explanation of the earlier results.